r5051 needs unit tests, documentation

[Mike Bayer]

for r5051:

• test that synonym and column_property can accept custom comparator
• test within declarative that synonym() may be used without explicitly specifying a descriptor (the use case is that synonym() has a custom comparator)
• add new features to CHANGES
• possibly add brief example of explicit column_property usage to declarative docs

[Mike Bayer]

75e8350e4d9c0a2631d11ea5de3dd1d1460690ce

[Mike Bayer]

Removing milestone: 0.5.0 (automated comment)